A typical subsea wellhead assembly has a high pressure wellhead housing supported in a lower pressure wellhead housing and secured to casing that extends into the well. One or more casing hangers land in the wellhead housing, the casing hanger being located at the upper end of a string of casing that extends into the well to a deeper depth. A string of tubing extends through the casing for conveying production fluids. A Christmas or production tree mounts to the upper end of the wellhead housing for controlling the well fluid. The production tree is typically a large, heavy assembly, having a number of valves and controls mounted thereon.
One type of tree, sometimes called “conventional” or “vertical”, has two bores through it, one of which is the production bore and the other is the tubing annulus access bore. In this type of wellhead assembly, the tubing hanger lands in the wellhead housing. The tubing hanger has two passages through it, one being the production passage and the other being an annulus passage that communicates with the tubing annulus surrounding the tubing. Access to the tubing annulus is necessary, both to monitor and bleed down pressure during production and to circulate fluids down the production tubing and up through the tubing annulus, or vice versa, to either kill the well or circulate out heavy fluid during completion. After the tubing hanger is installed and before the drilling riser is removed for installation of the tree, plugs are temporarily placed in the passages of the tubing hanger. The tree has isolation tubes that stab into engagement with the passages in the tubing hanger when the tree lands on the wellhead housing. This type of tree is normally run on a completion riser that has two strings of conduit. In a dual string completion riser, one string extends from the production passage of the tree to the surface vessel, while the other extends from the tubing annulus passage in the tree to the surface vessel. The plugs are retrieved on wireline through the completion riser, then the completion riser is retrieved. While workable, it is time consuming, however to assemble and run a dual string completion riser. Also, drilling vessels may not have such a completion riser available, requiring one to be supplied on a rental basis.
In another type of tree, sometimes called “horizontal” tree, there is only a single bore in the tree, this being the production passage. The tree is landed before the tubing hanger is installed, then the tubing hanger is lowered and landed in the tree. The tubing hanger is lowered through the riser, which is typically a drilling riser. A wireline plug is run through the tubing hanger running string and installed in the tubing hanger. After removal of the tubing hanger running tool, an internal tree cap is lowered through the drilling riser and installed in the bore of the tree. Access to the tubing annulus is available through choke and kill lines of the drilling riser. The tubing hanger does not have an annulus passage through it, but a bypass extends through the tree to a void space located above the tubing hanger. This void space communicates with the choke and kill lines when the blowout preventer is closed on the tubing hanger running string. In this system, the tree is run on drill pipe, which prevents the drilling rig derrick of the floating platform from being employed on another well while the tree is being run. This is also the case for the “conventional” tree, when installed on completion riser or drill pipe.
In another and less common type of wellhead system, a concentric tubing hanger lands in the wellhead housing in the same manner as a conventional wellhead assembly. The tubing hanger has a production passage and an annulus passage. However, the production passage is concentric with the axis of the tubing hanger, rather than slightly offset as in conventional tubing hangers. The tree does not have a vertical tubing annulus passage through it, thus a dual bore completion riser is not required. Consequently the tree may be run on a monobore riser. A tubing annulus valve is located in the tubing hanger since a plug cannot be temporarily installed and retrieved from the tubing annulus passage with this type of tree.
Normally, the tubing annulus valve is a check valve that prevents upward flow that might occur through the tubing annulus but allows downward flow. A disadvantage is that one cannot readily test a tubing annulus check valve to determine whether or not it is properly closing. A tubing annulus valve that is hydraulically actuated and which could be tested from above is desireable. However, these typically require hydraulic passages in the tubing hanger, which take up space and add complexity to the tubing hanger, rendering the designs potentially unreliable due to space restrictions.
During subsea well drilling, the floating platform may complete only one well at a time for production. However, in some instances, a platform might drill and case a number of nearby wells, and defer running the production trees until later. The production trees may be ran by the same platform or another. There have been instances where a tree was run by a lift line by a vessel onto a wellhead housing previously installed by another vessel. Generally, however, trees are run either on a completion riser or on drill pipe because they are large and very heavy. Both of these procedures require a derrick and drawworks. Drilling a well or running tubing also requires a derrick and drawworks, and typically, a floating platform has only one. Being unable to run a production tree from a platform at the same time that the platform is drilling or completing another slows field development.